Let it go into dormancy, prune it back and place it in a garage or under your house, anywhere to protect it from frost. Water very little. Treat it just about as you would a fuchsia or geranium. Bring it out in the spring and slowly re-introduce it to the sun. There are several sites devoted to brugmansia on the web.

In addition I would also like to say: take you some cuttings as well and place them in a glass with a small amount of water maybe 2" and let it sit in your window. Change the water frequently and plant it in the spring.
Darlene